Shrinkage Ring - Measuring the Restrained Shrinkage

A sample of freshly mixed mortar or concrete is compacted in a circular mold around an instrumented steel ring. Four (4) Wheatstone bridge strain gauges measure the compressive strain developed in the steel ring caused by shrinkage of the mortar/concrete specimen. Cracking of the test specimen is indicated by a sudden decrease in the strain of the steel ring. The age at cracking and the rate of tensile stress development are indicators of the material’s resistance to cracking under restrained shrinkage.

As specified in ASTM C1581, Significance and Use:

“This test method is for relative comparison of material and is not intended to determine the age at cracking of mortar or concrete in any specific type of structure, configuration, or exposure. This method is applicable to mixtures with aggregates of 13 mm/0.5” maximum nominal size or less. This test method is useful for determining the relative likelihood of early-age cracking of different cementitious mixtures and for aiding in the selection of cement-based materials that are less likely to crack under restrained shrinkage. Actual cracking tendency in service depends on many variables including type of structure, degree of restraint, rate of property development, construction and curing methods, and environmental conditions. This test method can be used to determine the relative effects of material variations on induced tensile stresses and cracking potential. These variations can include, but are not limited to, aggregate source, aggregate gradation, cement type, cement content, water content, supplementary cementing materials, or chemical admixtures. For materials that have not cracked during the test, the rate of tensile stress development at the time the test is terminated provides a basis for comparison of the materials.”